linux-kernel - Re: [PATCH] x86/ioperm: Use atomic64_inc_return() in ksys

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Message-ID: <be62f9c4-eca2-46bb-b566-77c0cbe1f15b@intel.com>
Date: Thu, 24 Oct 2024 08:20:57 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Uros Bizjak <ubizjak@...il.com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/ioperm: Use atomic64_inc_return() in ksys_ioperm()

On 10/7/24 01:33, Uros Bizjak wrote:
> Use atomic64_inc_return(&ref) instead of atomic64_add_return(1, &ref)
> to use optimized implementation and ease register pressure around
> the primitive for targets that implement optimized variant.

Ease register pressure at the end of a syscall?

I'll accept that we're doing this just as a matter of hygiene.  But it's
a stretch to say there are any performance concerns whatsoever at the
end of the ioperm() syscall.

So what is the real reason for this patch?